Activation of the Postsynaptic Cell

A feature that makes postsynaptic integration possible is that in most neurons one excitatory synaptic event by itself is not enough to cause threshold to be reached in the postsynaptic neuron. For example, a single EPSP may be only 0.5 mV, whereas changes of about 15 mV

FIGURE 8-29

Intracellular recording from a postsynaptic cell during episodes when (A) excitatory synaptic activity predominates and the cell is facilitated, and (B) inhibitory synaptic activity dominates.

are necessary to depolarize the neuron's membrane to threshold. This being the case, an action potential can be initiated only by the combined effects of many excitatory synapses.

Of the thousands of synapses on any one neuron, probably hundreds are active simultaneously or close enough in time so that the effects can add together. The membrane potential of the postsynaptic neuron at any moment is, therefore, the resultant of all the synaptic activity affecting it at that time. There is a depolarization of the membrane toward threshold when excitatory synaptic input predominates, and either a hyper-polarization or stabilization when inhibitory input predominates (Figure 8-29).

Let us perform a simple experiment to see how EPSPs and IPSPs interact (Figure 8-30). Assume there are three synaptic inputs to the postsynaptic cell: The synapses from axons A and B are excitatory, and the synapse from axon C is inhibitory. There are laboratory stimulators on axons A, B, and C so that each can be activated individually. An electrode is placed in the cell body of the postsynaptic neuron and connected to record the membrane potential. In Part 1 of the experiment, we shall test the interaction of two EPSPs by stimulating axon A and then, after a short time, stimulating it again. Part 1 of Figure 8-30 shows that no interaction occurs between the two EPSPs. The reason is that the change in membrane potential associated with an EPSP is fairly short-lived. Within a few milliseconds (by the time we stimulate axon A for the second time), the postsynaptic cell has returned to its resting condition.

In Part 2, we stimulate axon A for the second time before the first EPSP has died away; the second synap-tic potential adds to the previous one and creates a greater depolarization than from one input alone. This is called temporal summation since the input signals arrive at the same cell at different times. The potentials summate because there are a greater number of open ion channels and, therefore, a greater flow of positive ions into the cell. In Part 3, axon B is stimulated alone to determine its response, and then axons A and B are stimulated simultaneously. The two EPSPs that result also summate in the postsynaptic neuron; this is called spatial summation since the two inputs occurred at different locations on the same cell. The interaction of multiple EPSPs through ongoing spatial and temporal summation can increase the inward flow of positive ions and bring the postsynaptic membrane to threshold so that action potentials are initiated (Part 4).

So far we have tested only the patterns of interaction of excitatory synapses. Since EPSPs and IPSPs are due to oppositely directed local currents, they tend to cancel each other, and there is little or no change in membrane potential (Figure 8-30, Part 5). Inhibitory potentials can also show spatial and temporal summation.

Recording microelectrode

Inhibitory synapse *c

Recording microelectrode

Inhibitory synapse *c

Excitatory synapses

Axon^

I 0

Temporal summation

Spatial summation

râ

c o a. e

----------

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1

Threshold

S

Ii II

FIGURE 8-30

Interaction of EPSPs and IPSPs at the postsynaptic neuron. Arrows indicate time of stimulation.

Via the local current mechanisms described earlier, the plasma membrane of the entire postsynaptic cell body and the initial segment reflect the changes at the postsynaptic membrane. The membrane of a large area of the cell becomes slightly depolarized during activation of an excitatory synapse and slightly hy-perpolarized or stabilized during activation of an inhibitory synapse, although these graded potentials will decrease with distance from the synaptic junction (Figure 8-31).

In the previous examples, we referred to the threshold of the postsynaptic neuron as though it were the same for all parts of the cell. However, different parts of the neuron have different thresholds. In many cells the initial segment has a lower threshold (that is, much closer to the resting potential) than the threshold of the cell body and dendrites. In these cells the initial segment reaches threshold first whenever enough EPSPs summate, and the resulting action potential is then propagated from this point down the axon (and, sometimes, back over the cell body and dendrites).

The fact that the initial segment usually has the lowest threshold explains why the location of individual synapses on the postsynaptic cell is important. A synapse located near the initial segment will produce a greater voltage change there than will a synapse on the outermost branch of a dendrite because it will expose the initial segment to a larger local current. In fact, some dendrites use propagated action potentials over portions of their length to convey information about the synaptic events occurring at their endings to the initial segment of the cell.

Postsynaptic potentials last much longer than action potentials. In the event that cumulative EPSPs cause the initial segment to still be depolarized to threshold after an action potential has been fired and the refractory period is over, a second action potential will occur. In fact, as long as the membrane is depolarized to threshold, action potentials will continue to arise. Neuronal responses at synapses almost always occur in bursts of action potentials rather than as single isolated events.

Initial segment

(a) Excitatory synapse

(a) Excitatory synapse

Initial segment

(b) Inhibitory synapse

FIGURE 8-31

Comparison of excitatory and inhibitory synapses, showing current direction through the postsynaptic cell following synaptic activation. (a) Current through the postsynaptic cell is away from the excitatory synapse, depolarizing the initial segment. (b) Current through the postsynaptic cell hyperpolarizes the initial segment.

Initial segment

(b) Inhibitory synapse

FIGURE 8-31

Comparison of excitatory and inhibitory synapses, showing current direction through the postsynaptic cell following synaptic activation. (a) Current through the postsynaptic cell is away from the excitatory synapse, depolarizing the initial segment. (b) Current through the postsynaptic cell hyperpolarizes the initial segment.

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.